1. Field of the Invention
This invention relates to brazing metals composed of cobalt-chromium-palladium-based alloys containing transition metals such as cobalt, nickel, tungsten, molybdenum and certain metalloids; and more particularly to multicomponent alloys containing cobalt, chromium, palladium, nickel, tungsten, molybdenum, boron, and silicon, which are especially useful for brazing metals at high temperatures to produce high strength, high oxidation and high temperature and corrosion-resistant brazements. Alloys of the present invention have a composition represented by the formula: EQU Cr.sub.a Ni.sub.b W.sub.c Pd.sub.d Si.sub.e B.sub.f CO.sub.bal.
(plus incidental impurities), where the subscripts "a", "b", "c", "d" "e", and "f" are in atomic percent and "a" is in the range of about 15 and about 22, "b" is between about 0 and about 20, "c" is in the range of about 1 to about 5, "d" is between about 1 and about 10, "e" is in the range of about 5 and about 12, and "f" is between about 5 to about 12 and "bal" is the balance amount to total 100 percent.
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, brazing is accomplished by interposing a filler metal that has a melting point lower than that of the parts to be joined to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing filler metal. Upon cooling, a strong, preferably high oxidation and high temperature and high corrosion resistant joint is formed.
A few classes of products produced by brazing processes are used as critical parts of power turbines which are operated, for example as jet engines in the aerospace industry and in stationary power plants to generate electrical energy. Particular power turbine parts, such as turbine seals, first-stage turbine nozzle guide vanes, and turbine blades, are subjected to high temperature highly oxidized environments in operation. Thus, the brazed parts used in these applications must be able to withstand such harsh operating conditions in order to achieve high energy efficiency that directly relates with operating temperature.
An another important application of brazing technology is the manufacture of light-weight high temperature resistant honeycomb structures for leading edges of wings and other body parts of supersonic jets and reusable shuttles. In these applications, the base metals to be joined are mostly nickel- and cobalt-based superalloys and high chromium containing iron-based alloys. Such superalloys and iron-chromium-based alloys have complex compositions comprised of some or all of a group of transition elements such as cobalt, nickel, chromium, iron, and some refractory elements. Additionally, all these alloys also typically contain aluminum, titanium and, sometimes, yttria additions to improve their high temperature and high oxidation resistance. The latter is achieved due to intrinsic formation of oxide alumina/titania surface protecting film on such base metal parts.
Of particular importance for all parts subjected to high temperature service environment is their resistance to oxidation while maintaining the part's mechanical integrity. The oxidation resistance of these base metals is due to existence of the above mentioned dense alumina/titania protecting film on the part surface. Unfortunately, brazing by using filler metals containing active metalloid elements such as boron and silicon, causes a partial or even complete dissolution of these protecting oxide films in the brazed areas. As a result, the brazed interfaces act as conduits for oxygen penetration which can cause catastrophic part oxidation. Therefore, during the brazing of materials it is of paramount importance to preserve the integrity of the braze interfaces even if these oxide films cannot be preserved in the initial state.
Previously, some amorphous brazing filler metals consisting of cobalt/nickel-chromium-based alloys have been developed which exhibit a sufficient strength and good corrosion resistance at elevated temperatures. Such alloys have been disclosed, for example, in U.S. Pat. Nos. 4,260,666, 4,515,868, 4,515,869, 4,515,870, and 4,801,072. The alloys disclosed in these patents, however, each exhibit drawbacks, which make them unsuitable for brazing products that require prolonged service life at high temperature and in highly oxidizing and corrosive environments. For example, the alloys disclosed in U.S. Pat. No. 4,260,666, 4,515,868 and 4,801,072 contain the transition and refractory elements and boron and silicon. Unfortunately, boron due to its very small atomic radius diffuses extensively out of the joint area into alloys, particularly in those containing chromium, because of tendency to form strong chromium borides. These borides are formed preferentially at grain boundaries resulting in alloy brittleness and excessive oxidation or even complete failure. At the same time, these alloys contain no elements which protect the base metal from boron diffusion.
Regarding the multicomponent alloys disclosed in U.S. Pat. Nos. 4,515,869 and 4,515,870, they also contain the similar transition and refractory elements and boron and silicon but are based on nickel. Therefore these multicomponent alloys, contain only a moderate (less than 30 atom per cent) amount of cobalt and as a result are is insufficient to protect brazed parts from high temperature and highly oxidizing environment.
For the above reasons, the alloys previously known are not effective for use in brazed products to be employed in high temperature, high oxidizing and high stress environments existing in turbine engines and supersonic airspace structural applications.
Accordingly, there remains a need in the art for improved brazing filler materials suitable for brazing superalloys and iron-chromium-based alloys at high temperatures that can withstand a service in high temperature and highly oxidizing environments under high stresses for a long time.
Specifically, there has been a need in the art for a brazing filler metal that would form naturally a protecting layer of high temperature resistant phases at the braze interface, protecting base metal parts from excessive boron penetration on completing of brazing. Moreover, it would be even more beneficial if this layer could keep boron inside the joint preventing it from excessive diffusion into the base metal. Thus, it is an object of the present invention to provide such a brazing metal.
It is a further object of the present invention to provide brazing filler metals that first, contain major metallic elements that are compatible with high temperature resistant base metals; second, can wet oxide covered surface during brazing operation; and third, contain an element or elements that predominantly migrate to and form a protecting phase layer at the joint interface.